1. Field of the Invention
The present invention relates to a recording/reproducing system which uses an optical disk. More particularly, it relates to,.for example, an optical disk system which makes it possible to record video software on a writable optical disk, such as of the write once type or the rewritable or erasable type, and to reproduce both the recorded writable optical disk and a read only type optical disk, such as an LD (laser disk), by means of the same player or reproducer.
2. Related Art
VTRs (video tape recorders) employing magnetic tapes form the mainstream of recording/reproducing systems for video images. The VTR has the merit that users thereof can produce video software by themselves.
On the other hand, a recording/reproducing system employing an optical disk is, for example, what is called an "LD system" which uses an LD.
The LD which is commercially available has the video signal, audio signal and control signal of color television recorded on both its surfaces for a program length of one hour to two hours. This LD is fabricated in the following way. First, a stamper is prepared from a photoresist master on which the signals have been recorded. Subsequently, an acrylic resin is extruded using the stamper. A single-sided disk is obtained by coating the resulting acrylic resin plate with a reflective film and a protective film. When two such disks are fastened together, the LD is completed. Incidentally, the recording of the signals on the photoresist master is done as stated below. A video signal and a two-channel audio signal are respectively converted into frequency-modulated signals of unequal carrier frequencies. These signals are simply added, and the resulting signal is shaped into a square wave by a limiter. While an optical modulator is being controlled with the square wave, pits are formed in the surface of the photoresist master. The details of such an LD system are contained in "Guide to Video Disk and DAD" edited by Soichi Iwamura, issued by Corona Publishing Co., Ltd., pp. 122-142.
When compared with the VTR, the optical disk system as stated above has the merits of being easy to randomly access, affording a high picture quality, etc. However, the optical disk in the system is usually a read only type disk, and users of the system have difficulty producing video software by themselves. In order to eliminate the difficulty, write once type and erasable type optical disks have been proposed in "Video .alpha." July 1990, pp 73-78.
Besides, regarding a system for musical CD's (compact disks), a write once type CD which can be reproduced by a CD player or reproducer on the market has been employed, and users of the CD system have already been able to produce CD software by themselves.
With the write once type or erasable type optical disk, users of the optical disk system can produce video software by themselves. Nevertheless, there is the problem that such an optical disk is incompatible with the read only type optical disk because of different recording formats. The compatibility of the write once type or erasable type with the read only type is important in order that users may produce their own video software and that they may effectively utilize a large video software stock of read only type optical disks. Moreover, it would increase the cost of an optical disk player or reproducer whose recording format is different from that of the read only optical disks.
In the case of the write once type CD for musical use, the above problem is eliminated. However, even when this disk is intended to be applied to the video optical disk (LD), a high picture quality as afforded by the LD is not attained on account of a low C/N (carrier-to-noise) ratio.